1. Field of the Invention
The present invention relates to a method for controlling a radio-frequency device for a magnetic resonance tomography system, as well as to a magnetic resonance tomography system having such a radio-frequency device, and to a corresponding radio-frequency (RF) control device for executing such a method.
2. Description of the Prior Art
In the field of medicine, in recent years an imaging based on modality, detection and measurement of the nuclear spins of protons in a region of the body. The imaging is essentially carried out in three steps. First, a strong, stable, homogenous magnetic field is generated around the body region, producing a stable (aligned) orientation of the protons in the relevant bodily region. This stable orientation is then altered by supplying electromagnetic RF energy. Thirdly, this energy stimulation is terminated, and the nuclear resonance signals that have arisen in the body are measured using suitable reception coils in order to obtain information about the tissue in this part of the body. A system for carrying out an imaging method of this sort is also known as a magnetic resonance tomography system.
A magnetic resonance tomography system has a large number of components that operate together, each of which requires the use of modem, expensive-technologies. A central component of a magnetic resonance tomography system, to which the present invention relates, is the RF device. This device is responsible in particular for producing the RF pulses that are to be radiated into a body region under examination.
The RF pulses emitted by an RF power amplifier device of a magnetic resonance tomography system are conducted, via a measurement device, to a transmission coil that radiates the RF pulses into the body region. The term “transmission coil” is used herein to refer to any antenna device by which the RF pulses can be radiated.
With the development and establishment of magnetic resonance tomography systems, limit values for ensuring patient safety have been standardized that regulate the maximum RF radiation into a human body. A typical limit value for this is the maximum permissible SAR (Specific Absorption Rate) value.
In order to maintain these limit values, a measurement device acquires measurement values that represent the radiated power of the RF pulses radiated by the transmit coil. On the basis of a number of power measurement values, power control values are formed. These power control values are then compared with a rigid power limit value predetermined by a standard, this limit value being selected such that the predetermined SAR limit value is not exceeded. The RF device is then automatically limited in its functioning if a control value exceeds the predetermined threshold value.
That is, conventionally the maximum permissible SAR is converted to a maximum permissible power, and this power limit value is monitored. The physiological effect of RF energy on a human or animal body depends, inter alia, on the frequency and on the type of coil, i.e., on whether the coil radiates in e.g. a circular or linear polarized fashion, or whether it is for example a volume coil or a surface coil. In addition, the effect also depends on the position of the coil in relation to the body of the patient. For these reasons, in the conventionally monitoring it has been necessary in part to work with large safety margins from the actual critical value, in order to be able to ensure 100% safety for the patient when fundamental parameters, such as for example the position, are changed. This results in the permissible power limit value in general being considerably lower than the value that is actually necessary to produce the maximum loading.
Because, as a rule, lower RF power also results in a lower image quality, it is desirable to reduce this excessive safety margin. It should also be noted that a lower image quality has the end result that exposures may not offer the desired diagnostic possibilities, or even that exposures may have to be retaken, resulting in a higher total of exposure to the patient.
Another problem in such known monitoring is that, due to the dimensioning of the tomography system, the window that is visible during a measurement, known as the field of view, is limited. In order to produce a whole-body exposure of a person in one measurement, the person must be moved through the magnet during the magnetic resonance measurement. That is, the person is moved relative to the RF field radiated by the transmission coil, hereinafter called the “transmission field.” During this process, different body regions of the person are successively exposed to the RF radiation. The power radiated by the RF device therefore results in different specific absorption rates. That is, during the course of the measurement of a moving subject of examination, the absorption rates are not constant, contrary to the premise underlying conventional monitoring wherein the patient is situated in a particular position relative to the RF antenna during a measurement. Just as the specific absorption rate itself changes with the position of the subject of examination relative to the RF antenna, the SAR limit values also change with the patient position.
In conventional monitoring methods, neither current changes in the specific absorption rate nor current changes in the SAR limit values are able to be taken into account during the measurement.
In order to maintain the SAR limit values without the use of excessive safety margins, in DE 101 50 137 A1 and DE 101 50 138 A1 methods are proposed in which current SAR values that are to be expected are calculated before a measurement is carried out, on the basis of patient data, the position of the patient relative to the transmission antenna, and the planned measurement parameters. The parameters are then altered as necessary until the SAR values are within the SAR limit values. Here, the determination of the current SAR values takes place by a comparison of the current measurement situation with various measurement situations that are pre-specified in a database, for which SAR values calculated in advance are stored. As the current SAR value, the stored SAR value of the measurement situation that comes closest to the current measurement situation is used. Thus, in this method it is determined ahead of time, for the entire measurement, which RF power is permitted to be emitted to the patient at which point in time. In such a method, the controlling of the SAR value is not autonomous, but relies on instructions based on predictions of the measurement program that determines the pulse sequences for the measurement. Unforeseen events that, for various reasons, result in a sudden alteration in the measurement sequence cannot be taken into account. In particular, in this method it is not possible to move the patient through the tomograph during the measurement. An exposure of the whole body takes place by carrying out measurements at various fixed positions and later combining the image data from the various fields of view.